Recent work in the field of olefin upgrading has resulted in a catalytic process for converting lower olefins to heaver hydrocarbons. Heavy distillate and lubricant range hydrocarbons can be synthesized over ZSM-5 type catalysts at elevated temperatures and pressure to provide a product having substantially linear molecular conformations due to the ellipsoidal shape selectivity of certain medium pore catalysts.
Conversion of olefins to gasoline and/or distillate products is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 (Givens, Plank, and Rosinski) wherein gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of a ZSM-5 type zeolite. Particular interest is shown in a technique developed by Garwood, et al, as disclosed in European Patent Appication No. 83301391.5, published Sept. 29, 1983. In U.S. Pat. Nos. 4,150,062; 4,211,640 and 4,227,992 Garwood et al disclose the operating conditions for the Mobil Olefin to Gasoline/Distillate (MOGD) process for selective conversion of C.sub.3 + olefins to mainly aliphatic hydrocarbons.
In the process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization using a medium pore shape selective acid crystalline zeolite, such as ZSM-5 type catalyst, process conditions can be varied to favor the formation of hydrocarbons of varying molecular weight. At moderate temperature and relatively high pressure, the conversion conditions favor C.sub.10 + aliphatic product. Lower olefinic feedstocks containing C.sub.2 -C.sub.8 alkenes may be converted; however, the distillate mode conditions do not convert a major fraction of ethylene. A typical reactive feedstock consists essentially of C.sub.3 -C.sub.6 mono-olefins, with varying amounts of nonreactive paraffins and the like being acceptable components.
The above-described shape-selective oligomerization, as it applies to the conversion of C.sub.2 -C.sub.10 olefins over ZSM-5, is known to produce higher olefins up to C.sub.30 and higher. As reported by Garwood in Intrazeolite Chemistry 23, (Amer. Chem. Soc., 1983), reaction conditions favoring higher molecular weight product are low temperature (200.degree.-260.degree. C.) (390.degree.-500.degree. F.), elevated pressure (about 2000 kPa or greater), and long contact time (less than 1 WHSV). The reaction under these conditions proceeds through the acid-catalyzed steps of (1) oligomerization, (2) isomerization-cracking to a mixture of intermediate carbon number olefins, and (3) interpolymerization to give a continuous boiling product containing all carbon numbers. The channel systems of ZSM-5 type catalysts impose shape-selective constraints on the configuration of the large molecules, accounting for the differences with other catalysts.
The following model reaction path for propylene is set forth for purposes of explanation, and it should be taken as a theoretical path, as the process is presently understood by workers in the field. ##STR1##
The desired oligomerization-polymerization products include C.sub.10 + substantially linear aliphatic hydrocarbons. The ZSM-5 catalytic path for propylene feed provides a long chain with approximately one lower alkyl (e.g., methyl) substituent per 5 or more carbon atoms in the straight chain. The lubricant range final product can be depicted as a typical linear molecule having a sparingly-sutstituted (saturated) long carbon chain, as follows: ##STR2##
The final molecular conformation is influenced by the pore structure of the catalyst. For the higher carbon numbers, the structure is primarily a methyl-branched straight olefinic chain, with the maximum cross section of the chain limited by the pore size constraints of ZSM-5. Although emphasis is placed on the normal 1-alkenes as feed stocks, other lower olefins such as 2-butene or isobutylene, are readily employed as starting materials due to rapid isomerization over the acidic zeolite catalyst. Other mixed olefin rich streams can also be employed such as FCC off gas or Fischer Tropsch products. At conditions chosen to maximize heavy distillate and lubricant range products (C.sub.20 +) the raw aliphatic product is essentially mono-olefinic. Overall branching is not extensive, with most branches being methyl at about one branch per five or more atoms.
The viscosity index of a hydrocarbon lube oil is related to its molecular conformation. Extensive branching in a molecule usually results in a low viscosity index. It is believed that two modes of oligomerization/polymerization of olefins can take place over acidic zeolites such as HZSM-5. One reaction sequence takes place at shape constrained Bronsted acid sites inside the channels or pores, producing essentially linear materials. The other reaction sequence occurs on the outer surface, producing highly branched material. By decreasing the surface acid activity (surface .alpha.- value) of such zeolites, fewer highly branched products with low VI are obtained. Thus, U.S. Pat. No. 4,520,221 is concerned with the conversion of olefins to a 650.degree. F.+ lube fraction by contacting the olefins over ZSM-5 or related catalyst at elevated temperatures and pressures wherein the catalyst is treated with a bulky alkylpyridine or other basic compound whereby the surface activity or acidity of the catalyst is removed or substantially eliminated.
Also, U.S. Pat. No. 4,547,613 discloses forming a lube oil by contacting olefins with ZSM-5 or related zeolite which has been conditioned by previous contact with a light olefin.
Other patents which disclose the conversion of olefins over zeolites such as ZSM-5 in order to produce higher boiling products such as 650.degree. F.+ lube fractions include U.S. Pat. No. 4,517,399 which discloses contacting the olefins with ZSM-5 or related zeolite having a crystalline size greater than 2 microns; U.S. Pat. No. 4,126,644 which discloses the conversion of a C.sub.5 -400.degree. F. liquid fraction from a Fischer-Tropsch synthesis, predominately C.sub.5 -C.sub.10 olefins; and U.S. Pat. No. 3,322,848 which is directed towards the manufacture of high VI, low pour point lube oils from C.sub.10 -C.sub.18 normal alpha olefins by processing over crystalline aluminosilicates other than those related to ZSM-5.
Another catalyst found useful in forming lubricants from the polymerization of olefins having between 5 and about 14 carbon atoms per molecule is ditertiary alkyl peroxide catalyst such as disclosed in U.S. Pat. No. 2,937,129.
Commonly assigned U.S. Ser. No. 709,143, filed Mar. 7, 1985, discloses a two stage olefin to lube conversion process wherein a lower olefin feed is oligomerized in a primary stage with a medium pore shape-selective siliceous zeolite catalyst having acid cracking activity, and constraint index of about 1 to 12 and wherein the zeolite surface is rendered substantially inactive for acidic reactions by chemisorption of a surface deactivating agent; and at least a portion of the primary stage effluent is converted in a secondary reactor stage with an acid catalyst (BF.sub.3) to produce a high viscosity index lubricant range hydrocarbon.
One limitation of olefin oligomerization such as the MOGD process for producing lubes has been that higher viscosity lubes are not easily obtainable. A primary object of the present invention is to produce high viscosity lubes which have a low pour point and high viscosity index.